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JPEG Compression Basics
Posted By On June 6, 2005 @ 10:20 AM In Digital Photography | Comments Disabled
As you probably know, JPEGs are the most common type of file format for digital photography. They do a fantastic job at compressing an image and maintaining good quality. For more on image formats, check out:
Now, when you go to save a file as a JPEG, your imaging software may give you some options—that’s what we’re going to look at today.
The first and most common option is the ever-exciting quality setting. It’s usually pretty easy to understand—the higher the number, the better your image will look, but the bigger it will be.
How high you set this will depend on your usage. If you want the best quality, use a higher setting and live with a larger file size. If you’re short on space, try one of the medium settings. If you need to send the image to someone on dial-up, be kind and use one of the lower settings. Only you can decide how much quality is “good enough”.
Let’s look at how much quality is really lost when you go with lower settings. Let’s take this image:
Now, here’s a close-up of a window at high, medium, and low quality:
1. Highest quality (12):
2. Medium Quality (6):
3. Low Quality (2):
Although the low quality doesn’t look so great (still not too bad though), there really isn’t much difference between the highest setting and the medium setting. So, keep this in mind next time you’re trying to decide how much compression you need.
Also, note that some software will show you how big the finished file will be—really helpful If you’re sending it off in an e-mail or using it on a webpage.
OK, that’s the standard stuff, but many programs also offer you some format options. Here are the three most common:
Baseline (Standard) — This is your garden variety JPEG. It’s just saved as normal, nothing special, yawn.
Baseline Optimized — This is like the above, but you get a little more compression while retaining the same quality. Used to be some web browsers wouldn’t support this, but that pretty much went out with Netscape1.0.
Progressive — This is the coolest of the three, at least for web designers. Have you ever been on a website where you saw a picture download in “waves”? You know, at first it’s kind of chunky and blurry, then another pass goes by and it looks a little better, then finally it gets nice and sharp? That’s what this does.
If you have an option for “progressive”, then you probably also have an option for the number of scans—this is simply the number of “waves” the picture needs to go through in order to download.
That’s it! Probably more than you ever wanted to know about JPEGs!
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